1. Field of the Invention
The present invention relates to a developing method employing electrophotography, which can be applied to electrostatic image formation performed by photocopiers, printers, facsimile apparatuses, or the like., and relates to a developing device and an image forming apparatus using the developing method.
2. Description of the Related Art
Presently, with image forming apparatuses such as photocopiers, printers, facsimile apparatuses, or the like, a toner image is formed by an image carrying member (which will be referred to as a “photosensitive member” hereafter) including a photosensitive layer having photoconductivity on the surface, and a developing device storing a developing agent. In general, a two-component developing agent (which will be simply referred to as a “developing agent” hereafter) mainly formed of toner and magnetic carrier is employed for the developing device because it makes color image formation easy. The developing agent is mixed and stirred within the developing device so as to be charged due to friction. The toner electrostatically adheres to the surface of the magnetic carrier due to electrostatic charge. The magnetic carrier having the toner adhered to the surface thereof is magnetically attracted onto the surface of a developing agent carrying member (which will be referred to as a “developing sleeve” hereafter) including a magnet. Subsequently, the magnetic carrier is transported due to rotation of the developing sleeve.
The developing sleeve contains a magnet (which will be referred to as a “developing main magnet” hereafter) at a position which is the closest to the photosensitive member. The closer to the developing main magnet the transported developing agent is, the greater the number of magnetic carrier particles in the developing agent which forms chain along the magnetic lines of force is. The great number of chains looks like a brush, so, in general, are also referred to as a “magnetic brush, and the developing method using the magnetic brush is referred to as “magnetic-brush developing”.
It is assumed that with the magnetic-brush developing, the magnetic carrier serving as a dielectric substance increases the electric field strength generated at a gap between the photosensitive member and the developing sleeve, and the toner is transferred from the surfaces of the magnetic carrier particles positioned at the tips of the chains of the magnetic brush to the photosensitive member due to the increased electric field strength. Accordingly, with the conventional magnetic-brush developing, developing with toner is performed only at a portion where the magnetic brush is generated. Accordingly, the magnetic-brush developing with toner is performed using only a limited portion, leading to difficulty in performing developing with a large amount of toner while adjusting other conditions.
However, developing methods for obtaining a high-density image using the limited region have been proposed.
Japanese Patent No. 2,668,781, for example, describes a developing method that uses both of toner particles carried on the brush chains of magnetic particles and toner particles carried on the developing agent carrying member for development by using an alternating electric field. With the developing method using the alternating electric field, it is known that the toner which has temporarily adhered to the photosensitive member hops therearound, moves to a position around the middle portion of a dot or line, which is positioned deep within a latent image, and is fixed, thereby obtaining a smooth image without edge effects.
Further, another developing method is proposed that the toner is carried by the photosensitive member, following which excessive toner is removed, in order to obtain a high-density image with low fogging. For example, each of Published Japanese patent application Nos. 6-208304 and 7-319174 describes a developing device wherein the magnetic toner carried on the surface of an electrostatic latent image carrying member including a magnet is brought into contact with an electrode roller including a magnet, thereby removing excessive toner which adheres to portions other than the latent image portions.
Published Japanese patent application No. 5-46014 describes a developing device in which a first developing roller develops a latent image, and then a second developing roller supplied only with magnetic carrier particles removes excess toner particles.
However, the above-described proposed developing methods limit a developing region to a region where the magnetic carrier particles or the like come into contact with the photosensitive member. It is therefore difficult to achieve sufficiently high image density only with the toner particles carried on the chains of the magnetic carrier particles and the toner carried on the developing sleeve. Further, an electrode effect cannot easily implement a smooth high-quality image with high uniformity of a solid portion because the number of chains formed of the magnetic carrier particles or the like is small.
Accordingly, with the conventional developing devices, a developing agent with high-density toner is used so as to increase uniformity of a solid portion. However, in this case, developing is performed with an excessively high performance in a normal state, leading to a new problem that the horizontal/vertical ratio of a line may greatly deviate from 1. The horizontal/vertical ratio used here means the width of the line in the main-scanning direction as to the width of the line in the sub-scanning direction orthogonal to the main scanning direction.
As a method for obtaining a high-quality image which exhibits high image density in a solid portion, U.S. patent application Publication No. 2002-0146632A1 describes a developing method wherein a developing agent carrying member including a magnet is disposed opposite to an image carrying member, a two-component developing agent containing a toner and magnetic carrier is carried on the surface of the developing agent carrying member, a magnetic brush is formed at least in a region where the developing agent carrying member faces the image carrying member while causing the magnetic brush to flow by rotating the developing agent carrying member and the magnet with difference in speed, and free toner, which parts from the magnetic carrier particles in the flow, adheres to a latent image on the surface of the image carrying member, and thereby a toner image is formed. However, there is no mention of any method for obtaining an image without edge effects.
Thus, it is desirable to provide a developing method for obtaining a smooth high-quality image with high uniformity of image density on a solid portion without edge effects while suppressing the deviation of the horizontal/vertical ratio at a portion other than the solid portion.
It is further desirable to provide a developing device and an image forming apparatus using the developing method.